The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, the correction of acquired x-ray attenuation data using image reprojection.
In a current computed tomography system, an x-ray source projects a fan-shaped beam, which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" or "projection," and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data are processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reconstruct an accurate image from the attenuation measurements, the logarithm of the measurement should represent the sum, or line integral, of the linear attenuation coefficients along the x-ray beam. This would be true if a monoenergetic x-ray source were used but in practice, inaccuracies are introduced because broadband x-ray sources are used and the attenuation coefficients of tissues vary nonlinearly as a function of beam energy. "Beam hardening" corrections are made to the attenuation measurements to offset these errors, but these usually offset only first order estimates of the measurement errors.
An iterative beam hardening correction method has been proposed by P.M. Joseph, et al., "A Method for Correcting Bone Induced Artifacts in Computed Tomography Scanners," JCAT, Vol. 2, No. 1, pp. 100-108, 1978, which corrects for higher order beam hardening effects. However, this method requires that a modified version of the image be reprojected back to the separate views so that the corrections can be applied to the x-ray measurement made by each detector channel. Such reprojection of an image is very computation intensive, particularly when the image is acquired with a fan beam. There exists a need for faster and less complex fan beam reprojection techniques so that iterative beam hardening corrections can be employed in commercially available CT systems. Also, such reprojections are required in streak suppression methods such as that described by G. Henrich, "A Simple Computational Method For Reducing Streak Artifacts in CT Images," Computerized Tomography, Vol. 4, pp. 67-71, 1980; and in artifact removal methods such as that proposed for metal clips by G. H. Glover and N.J. Pelc, "An Algorithm for the Reduction of Metal Clip Artifacts in CT Reconstructions," Med. Phys., vol. 8, pg. 799, 1981.